In recent years attempts have been made to lower the building cost of ships by making hulls in a form as full as possible in relation in a required deadweight all as part of an improving approach to the ship's economy of structure and operation.
However, as the hull increases in fullness, the flow turbulence and the nonuniformity of flow field around the stern also increases, as a consequences an increase in horsepower is required due to the increase in resistance and there is thus a decrease in propulsive efficiency, an increase in propeller cavitation, vibration and/or noise level, which may offset any economic benefit attained by lowering the building costs. Accordingly, there has been need to remedy the lowering of ship's performance due to corpulence of hull form, while maintaining the initial building cost at a lower level.
Among the devices which have heretofore been applied with an intent to improve the performance of ships are, (A) ducted propeller (or so-called nozzle propeller) and (B) stern fin.
(A) The ducted propeller, in which a duct is provided around the propeller as shown in FIG. 16, exerts thrust not only by the propeller but also by the duct as the result of hydrodynamic interaction between the propeller and duct, and thereby improves propulsive efficiency.
This ducted propeller utilizes hydrodynamic interaction between the propeller and the duct, and therefore it is necessary to optimize the ducted propeller as an integral propulsor including the propeller and duct. Accordingly, conventional propellers without the duct can not be used as impellers for ducted propellers and, instead, a specially designed propeller is required which has a diameter and pitch different from the conventional propeller. Based upon previous studies about ducted propellers, it is known that the appropriate length of the duct is about 50% of the diameter of the propeller and that the appropriate position of the propeller is about the mid-position of the duct length. It is also known that the narrower the clearance between the propeller tip and the duct inner surface, the higher the propulsive efficiency will be.
As described above, by adopting the ducted propeller, it is possible to increase the propulsive efficiency and decrease the required horsepower of a ship with a heavily loaded propeller. However, in order to maximize the propulsive efficiency of the ducted propeller, the clearance between the inner surface of the duct and the propeller tip must be minimized, and under such a condition cavitation erosion is liable to occur on the inner surface of the duct due to the cavitation generated near the tip portion of the propeller on the blade surface thereof resulting in damage to the duct.
Moreover, if the duct is located too close to the ship hull, an increase of resistance (increase of thrust reduction fraction) and a drop of propulsive efficiency will be caused due to the acceleration effect of the flow around the ship hull owing to the duct. Accordingly, it is necessary that the leading edge of the duct be properly located away from the ship hull (the stern frame). Also, in the ducted propeller design, in order to prevent the duct erosion a shortened duct which is situated just in front of the propeller and aft of the stern frame is also considered, but in this case dramatic improvement of propulsive performance cannot be expected.
As described above, the ducted propeller requires a high degree of accurate in construction and is also subject to damage by cavitation erosion, and in order to avoid this, the propeller tip and the inner surface of the duct are designed so as to be sufficiently separated from one another, the propulsive efficiency is inevitably reduced below that of conventional propellers.
In the case where the ducted propeller is intended to be applied on an existing ship, if the existing propeller is utilized, the revolution speed of the propeller becomes too high or the main engine output corresponding to the rated revolution speed becomes too small, which inevitably necessitates replacement of the existing propeller by a new one, thus sacrificing the economy achieved.
(B) The stern fins are, as shown in FIG. 17, appendages from the ship hull mounted at the upper portion of the stern portion of the hull in front of the propeller. The stern fins are effective to reduce hull vibration and noise by suppressing the unstable flow around the stern, particularly in the area of the cruiser stern portion. However, the effective power savings is minimal and the so-called tip vortex is produced from the edge of fin, thus resulting in an increase of resistance in many cases.
Moreover, this tip vortex sometimes causes cavitation which flows down to the propeller located aftward, resulting in damage to the propeller and an increase in hull vibration. Further, the stern fins are of cantilever construction and, therefore, a substantial reinforcement of the ship hull is necessary for supporting the fins. Because of these difficulties, the stern fins can not be used effectively.